Sometimes, but not always, a source is part of a larger whole. MLA calls this larger whole the "Container." Basically, the Source is inside of a Container.
Visualize this idea by thinking of nesting dolls. With nesting dolls, you open a large doll to reveal a smaller one inside. Then you open that smaller doll to find an even smaller one inside of it!
Using this nesting doll analogy, the source is a smaller doll inside the larger doll.
When the source you are using is a part of a whole, it is inside a Container. We include the Title of Container as part of the works cited entry.
The Title of Container is the beginning of a complete sentence.
The Title of Container is written in Title Case, which capitalizes all the major words in the title. Each citation style has its own capitalization particulars, and MLA's are found in section 2.90 - 2.98 in the manual.
These examples show where the Title of Container fits inside the bigger works cited entry: